| BEN |
| BOTANICAL ELECTRONIC NEWS |
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| ISSN 1188-603X |
| No. 260 November 8, 2000 | aceska@freenet.victoria.tc.ca | Victoria, B.C. |
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More than three decades ago, a preliminary paper followed by a monograph (Jenik 1959, 1960) attempted to describe and explain the origin and unequal distribution of biodiversity in the Sudetes and other Hercynian/Variscan mountains in Europe. The essence of these publications, the theory of anemo-orographic systems (A-O systems for short), has become a reasonably feasible model for describing and explaining large-scale ecosystem patterns encountered in the summit areas of these "middle-mountains". Some geographers positively responded to this concept, which aims at integration of biogeographical knowledge and topoclimatic and geomorphologic evidence. Many ecologists and conservationists have utilized and even further developed this theory (see a review in Srutek 1990) and applied it to other European mountains (Jenik, 1990).
In Central Europe, the Hercynian mountains create an arch of separate massifs situated to the NW and N of the Alps. Their major representatives are the Vosges, Black Forest, Bohemian Forest, the Ore Mountains and the High Sudetes, the latter with three dominant elevations called Giant Mts. (1602 m), Snow Mts. (1423 m) and High Jesenik Mts. (1491 m). A product of Palaeozoic mountain-building, these massifs - despite various specificities - are similar in age, lithology, georelief, climate and, in their plant and animal life. They consist mostly of gneiss, crystalline schists and granite, and the base-rich carbonate rocks and volcanic rocks are scarce.
Due to advanced denudation in the Tertiary, their rounded georelief lacks rugged rock-faces, precipitous cliffs and deeply cut ravines. Seldom surpassing the altitude of 1500 m a.s.l., they are transformed by periglacial cryogenic processes or glacial erosion only in their topmost areas. In contrast to rugged landforms and glaciated valleys met with in the Alps or Carpathians, these middle-mountains, called also "forest-mountains" ("Waldgebirge" in German), offer only constrained areas for treeless ecosystems and fine-grained alpine biodiversity.
The surface of Hercynian mountains is largely covered by coniferous and mixed forests. A product of Postglacial succession, the flora and fauna consist of widely distributed "common" European species. In the German and Czech botanical literature, the expression "Hercynian flora" is a pejorative term indicating species-poor and monotonous floristic composition. Zonal forest communities consist of only a few dozens of vascular species, and similar uniformity refers to other life forms, such as mosses, lichens, fungi, and to many groups of vertebrate and invertebrate animals.
There exists, however, a small number of local exceptions, species-rich sites, which are recurrently reported in documents of European natural history.
These species-rich sites
For example, about 500 vascular species, coexisting in 30 different plant communities, have been recorded from a small valley-head called Velka Kotlina (Jenik et al. 1980). Species-rich fauna usually accompanies the botanical wealth. Occurrence of endemic and/or biogeographically and taxonomically isolated populations suggests that these localities serve as (i) refuges of retreating biota, (ii) forerunner sites of spreading biota, and even (iii) centres of active microevolution for new species (Jenik 1983).
Earlier explanations of outstanding biodiversity in these particular localities were rather inconsistent and confusing (Jenik 1961: 54-74). Many explanations linked the biotic wealth with a "sheltered site", a general qualification which was seldom described in terms of physical parameters. By many hypotheses, mainly locally outcropping base-rich rocks were taken into account. Detailed geological and geobotanical research did not confirm the primary role of this factor: some species-rich communities evidently occur within the acid parent rocks and, vice versa, the existence of mineral-rich rocks in the subsoil does not always support species-rich ecosystems.
In order to understand the identity of species-rich localities, broader environs of these sites must be taken into account (Soukupova et al., 1995). Though numerous arctic and alpine organisms participate, their refuges do not occupy the highest altitudes reached in particular ranges. In contrast, the majority of diversified ecosystems occurs at lower altitudes, often on treeless avalanche tracks below the present-day timberline. Remarkably, many arctic/alpine/nordic/boreal species of the Hercynian mountains coexist in association with numerous southern/lowland plants and animals.
A detailed analysis of the biodiversity pattern in the High Sudetes disclosed repeated geomorphological and meteorological components in these species-rich localities: (i) linkage to east-facing slopes, and situation on the eastern margin of large summit plateaux or saddles, (ii) situation on back side of an upland plateau or saddle, with regard to a funnel-shaped valley ascending the summit area and accelerating air currents from W, SW or NW, (iii) coincidence with recurrent snowdrifts, cornices and active avalanche action in winter, and (iv) coincidence within various concave landforms, such as nivation niches and glacial corries.
In terms of georelief, the high-biodiversity sites coincide mostly with E-facing (NE, E, SE) concave landforms, particularly with shallow hollows or deeply cut cirques, which are continually subjected to nivation and, in the past, were sculpted by glaciers. Remarkably, the topmost Hercynian peaks lack prominent glacial cirques even on their eastern flanks. If altitude lacks importance, what is then the general setting of the remarkable biodiversity centres? The Giant Mts., in the Czech and Polish territory offer a particularly meaningful picture: 15 cirques with botanical and zoological sanctuaries situated to the east of large upland plateaux. In the High Jesenik Mts. and in Black Forest two large funnel-shaped valleys join in the topmost area, and the famous cirques and species-rich habitats are situated at the eastern side of these junctions. In a range stretched from north to south, such as the Vosges, understandably, a variety of nivation hollows and cirques is developed along the long E-facing flank of the ridge.
Existing interplay of landforms and meteorological/ecological effects in culminating areas of the Hercynian mountains have been summarized in a generalized model called the Anemo-Orographic System.
According to the above quoted monograph (Jenik 1961), this model consists of a
Funnel-shaped windward section
Wind-accelerating summit section
Leeward turbulent section
A great variety of real A-O systems is encountered in different ranges and parts of these ranges. In the above quoted monograph (Jenik 1961: 195-196) various models have been proposed (1) according to their distance or proximity to the generalised model as "imperfect" or "perfect", respectively, and (2) according to occasional combination of two or several funnel-shaped windward valleys, as "simple" and "composed" A-O systems. Observations in the past four decades disclosed a number of less perfect A-O systems in numerous mountains of Europe. Wherever unilateral winds blow over suitably sculpted landforms, the respective windward, topmost and leeward sites develop in contrasting ecosystems which are clearly indicated by plant life. However, only long-term interaction of broader relief, wind action and plant succession results in the scenery of a "perfect" A-O system.
The late Prof. Askell Love, University of Colorado, was the first to indicate the potential applicability of A-O systems outside Europe, i.e., in the area of Mount Washington, White Mountains. This massif is marked by terminal situation in a funnel-shaped configuration of the Presidential Range, and by extremely unilateral western winds. Elevation, georelief with distinctive climate, position of the timberline and vegetation are similar to the European middle-mountains. Though covered by an ice-sheet in the Glacial period, the famous species-rich Alpine Garden, Tuckerman's Ravine, Huntington Ravine and Great Gulf were very likely developed and preserved due to their stabilised leeward position and avalanche action throughout the Postglacial period. The A-O system model seems to fit the spatio-temporal scale and evolutionary history of Mount Washington more satisfactorily than the "alpine mesotopographic gradient" described by Billings (1973) and illustrated in the new edition of "North American Terrestrial Vegetation" (reviewed in BEN No. 252).
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